Downhole fluid analysis is often used to provide information in real time about the composition of subterranean formation or reservoir fluids. Such real-time information can be advantageously used to improve or optimize the effectiveness of formation testing tools during a sampling processes in a given well (e.g., downhole fluid composition analysis allows for reducing and/or optimizing the number of samples captured and brought back to the surface for farther analysis). More generally, collecting accurate data about the characteristics of formation fluid(s) is an important aspect of making reliable predictions about a formation or reservoir and, thus, can have a significant impact on reservoir performance (e.g., production quality, volume, efficiency, etc.).
Fluid characteristics such as composition, density, viscosity, formation water or formation fluid resistivity, etc. are typically measured using formation fluid testers that are deployed via wireline tools and/or logging-while-drilling (LWD) tools, both types of which are commonly available. Formation fluid testers often use sensors that are in-line with a flowline of a formation fluid tester portion of a wireline or LWD tool and which may be at least partially in contact with or exposed to fluid(s) in the flowline.
Formation fluid resistivity is an important formation fluid characteristic because fluid resistivity characteristics can be advantageously used or applied in many ways. For example, formation fluid resistivity measurements can be used to determine formation water resistivity (Rw) which, in turn, can be used for petrophysical analysis of a formation. Also, for example, resistivity measurements can be used to determine the injection or formation water in water flood carbonates, determine whether carbonate transition zones have been invaded by formation water or water-based mud (WBM), determine salinity, quantify the level of WBM contamination, and/or perform a robust downhole fluid analysis at high temperature and high pressure.
Fluid resistivity measurement devices typically use either a direct measurement technique or an inductive measurement technique. Inductive fluid resistivity measurement devices induce a current in a portion of a flowline. In particular, inductive resistivity measurement devices typically employ two electrodes that are spaced along the flowline and are electrically insulated from each other (e.g., by an insulator or otherwise electrically insulated portion of the flowline). However, as is the case with many flowline sensors or measurement devices, the electrodes of an inductive resistivity measurement device are in direct contact with the fluid(s) flowing in the flowline. As a result, over time, one or both of the electrodes can become at least partially coated by substantially electrically insulating (i.e., substantially non-conductive) fluid(s) such as, for example, WBM, oil, etc. If one or both electrodes become contaminated with such non-conductive coatings, the resistivity measurements made by resistivity measurement device are biased or inaccurate.